Google Image Search, recently rebranded as Google Images, made it easier to change the SafeSearch filter by including a drop-down below the search box. The default option is "moderate", which excludes explicit images, but you can also disable SafeSearch or select the "strict" filter, which takes into account your keywords and the text from web pages.
Now you can also report offensive images that aren't filtered by SafeSearch. "Many users prefer not to have adult sites included in search results (especially if kids use the same computer). Google's SafeSearch screens for sites that contain explicit sexual content and deletes them from your search results. No filter is 100 percent accurate, but SafeSearch should eliminate most inappropriate material," explains Google.
The word "offensive" is pretty vague and Google doesn't define its scope, but you shouldn't use the new option for reporting irrelevant images or spam results.